The present invention relates to the field of scrubbers that reduce sulfur emissions from combustion gases of fossil fuels. More specifically, the present invention relates to selectively removing liquid phase metal from the process liquor produced by the scrubbers.
The combustion of coal, fuel oil, and petroleum coke in electric power plants produces sulfur dioxide (SO2) flue gas emissions. Left uncontrolled, the emission of high levels of sulfur dioxide into the atmosphere can result in a number of health problems, including respiratory impacts and aggravation of existing cardiovascular disease. In addition, moderate to high levels of sulfur dioxide emissions lead to acid deposition, which can result in degradation of visibility due to the formation of haze, acidification of lakes and streams, damage to the foliage of trees and agricultural crops, and degradation or destruction of buildings and monuments. Considering the health and environmental impacts, most countries place some limits on the allowable levels of uncontrolled sulfur dioxide emissions originating from electric generating facilities.
Power producers therefore are continually seeking cost-effective ways to minimize pollutant formation in the combustion process and to remove pollutants, once they are formed, from the flue gas. The removal of pollutants, such as sulfur dioxide, from the flue gas is typically achieved using flue gas desulfurization (FGD) systems. Wet FGD systems, also referred to as scrubbers, are designed to introduce an alkaline sorbent consisting of lime or limestone in a spray form into the flue gas exhausted by a coal-fired boiler. The alkaline sorbent reacts with the sulfur dioxide in the exhausted flue gas to form inert compounds, such as calcium sulfite (CaSO3) and calcium sulfate (CaSO4). The calcium sulfite or sulfate is allowed to settle out of the water used in the wet FGD scrubber and removed for disposal. Most of the water, also referred to as process liquor, is recycled.
The efficiency of a scrubber to remove sulfur dioxide is a saddle shaped equation. That is, the lowest scrubbing efficiency occurs when a scrubber is at forty to sixty percent oxidation. Oxidation refers to the degree to which the sulfur dioxide that is absorbed by the system is oxidized once it has become soluble in the liquid phase. When the scrubbing efficiency of the scrubber is in the range of forty to sixty percent oxidation, severe gypsum scaling can occur. The scaling limits system reliability and greatly increases maintenance costs.
To correct the problem of scaling, two types of processes to control oxidation have been developed. One process is inhibited oxidation. In this process, the degree to which absorbed sulfur dioxide is oxidized is controlled to a very low level by the addition of an additive to inhibit oxidation. A byproduct produced by inhibited oxidation is calcium sulfite, which is increasingly being used in the production of alpha plaster, a high compressive strength plaster of Paris. The additives commonly used to inhibit oxidation are thiosulfate and elemental sulfur, which reacts to generate thiosulfate.
Another process used to control oxidation is forced oxidation. This process uses air, typically sparged into the reaction or hold-tank of the scrubber by air blowers to maintain high and near-complete oxidation of absorbed sulfur dioxide. A reusable and saleable solid byproduct produced by forced oxidation is calcium sulfate, or gypsum, typically used for plaster, wall board, some cements, fertilizer, paint filler, ornamental stone, and so forth.
Although effective in substantially reducing sulfur from combustion gases, inhibited oxidation and forced oxidation wet scrubbers require a significant portion of a power plant""s electrical output, sometimes in the range of six to seven percent. In addition, a wet-scrubber uses thousands of gallons of water to operate. As such, a large installation may consume one hundred to two hundred million gallons of water a month.
In addition, the ability to recycle the process liquor used by a wet FGD scrubber is limited by the amount of large metals in solution in the process liquor. For example, when iron is present in the process liquor from an inhibited oxidation scrubber, the iron acts as an oxidation catalyst by destroying the thiosulfate presence. This leads to plugging of parts of the process, and less than optimum operation of the inhibited oxidation scrubber. Typically, the iron presence in the process liquor is dealt with by xe2x80x9cblowing downxe2x80x9d, or removing the process liquor, from the process and replacing the process liquor with fresh water. Unfortunately, blowing down the process liquor also removes chemicals, such as magnesium salts and alkalinity sources, considered valuable to the inhibited oxidation process.
In contrast, the presence of iron enhances the ability of a forced oxidation scrubber process to form calcium sulfate. However, aluminum in the process liquor undesirably impacts the efficiency of the forced oxidation scrubber. The presence of solution phase aluminum in the process liquor inhibits the ability of calcium to tie up with the absorbed sulfur dioxide to form calcium sulfate. Indeed, the presence of aluminum in the process liquor is a primary reason calling for the use of the air blowers in the forced oxidation process. These air blowers typically consume several megawatts of power.
Accordingly, it is an advantage of the present invention that a system and method are provided for removing a solution phase metal from process liquor produced by a flue gas desulfurization (FGD) scrubber.
It is another advantage of the present invention that a system and method are provided that achieve savings in terms of energy and water consumption by an FGD scrubber.
It is another advantage of the present invention that a system and method are provided that selectively remove a solution phase metal from the process liquor produced by either of an inhibited oxidation and a forced oxidation scrubber.
Another advantage of the present invention is that a system and method are provided that selectively replace the removed solution phase metal with a desired solution phase metal.
It is yet another advantage of the present invention that a system and method are provided that may be cost effectively implemented within an existing wet FGD scrubber.
The above and other advantages of the present invention are carried out in one form by an electrochemical cell system for removing a solution phase metal from process liquor. The electrochemical cell system includes a first half-cell having a first inlet configured to receive a first portion of the process liquor, and having a first outlet. A cathode at which a reduction reaction occurs with the first portion of the process liquor is in the first half-cell. The first portion of the process liquor is released from the first outlet following the reduction reaction. The electrochemical cell further includes a second half-cell having a second inlet configured to receive a second portion of the process liquor, and having a second outlet. An anode at which an oxidation reaction occurs with the second portion of the process liquor is in the second half-cell. The second portion of the process liquor is released from the second outlet following the oxidation reaction. An electrical circuit is coupled between the cathode and the anode. An ionic conductor section containing an ionic conductor enables a transfer of ions from the ionic conductor into each of the first and second half-cells. The reduction and oxidation reactions form a redox reaction causing the solution phase metal to be removed from one of the first and second portions of the process liquor.
The above and other advantages of the present invention are carried out in another form by a method for removing a solution phase metal from process liquor output from a scrubber. The method calls for establishing an electrochemical cell having a first half-cell, a second half-cell, and an ionic conductor section containing an ionic conductor for enabling a transfer of ions from the ionic conductor into each of the first and second half-cells. The first half-cell has a cathode located therein and the second half-cell has an anode located therein. The anode includes a metal having an electronegativity less than an electronegativity of the ionic conductor. The method further calls for receiving a first portion of the process liquor at the first half-cell, receiving a second portion of the process liquor at the second half-cell, and producing a redox reaction in the electrochemical cell system. The redox reaction causes the solution phase metal to be removed from one of the first and second portions of the process liquor. Following the redox reaction, the first and second portions of the process liquor are combined and the combined process liquor is returned to the scrubber.